Five Colombian soldiers were killed and 10 others were wounded Tuesday in an explosion, a statement from the army said.
The army called the explosion an act of terrorism and blamed rebels.
The statement said the incident took place in Norte de Santander, a northeastern region of the country bordering Venezuela.
The incident comes a day after the leftist rebel group, the National Liberation Army, or ELN by its Spanish initials, said it would carry out a unilateral ceasefire during legislative elections next month.
There was no immediate claim of responsibility.
The army called the actions cowardly, saying the ELN "employed unconventional methods and means of war, prohibited by international human rights, which puts at risk the lives of our troops and our community who transit that area."
The Colombian army passed along its condolences to the family members and friends "of our brave men who were murdered, who gave their lives for their duty."
Colombian President Juan Manuel Santos repudiated the attack on his Twitter account and said those responsible would be held accountable.
Last month, ELN took credit for two bombings at police outposts.